Paper mills have for many years made extensive use, for the cleaning of papermaking stock, of pressure screening apparatus embodying a cylindrical perforated screening member defining screening and accepts chambers on the opposite sides thereof in a closed housing, and including a rotor member which operates in one of the chambers to keep the screening perforations open and free from solid material having a tendency to cling to the screening surface. According to conventional practice, the stock or furnish is delivered to the screening chamber adjacent one end of the screening cylinder, and the material rejected by the screening cylinder is collected and discharged from the opposite end of the screening chamber.
The assignee of this invention has manufactured many such screens in accordance with a series of co-owned U.S. patents, commencing with Staege U.S. Pat. No. 2,347,716, and followed by Martindale U.S. Pat. No. 2,835,173, Seifert U.S. Pat. Nos. 3,849,302 and 4,105,543, Seifert-Chupka U.S. Pat. No. 3,970,548, Chupka-Seifert U.S. Pat. Nos. 4,155,841 and 4,383,918, Lehman U.S. Pat. No. 4,276,159, and Chupka et al U.S. Pat. Nos. 4,663,030 and 4,919,797. These patents demonstrate substantial detailed variation in screens of the above type, especially in the size, configuration and spacing of the perforations in the screening cylinder, as well as in rotor design, but in all of those patents, the screening member is a cylinder.
The art has also experimented, but to a considerably less extent, with screens for paper making wherein the perforated screening member comprises a flat annular plate, and in some instances, the combination of two such plates on opposite sides of a screening chamber has been proposed. An early example of such apparatus is shown in Cowles U.S. Pat. No. 2,180,080 wherein a pair of opposed perforated plates serve both as stationary refining members and screening members, in that a rotor operating between the two plates would rub the stock against the plates until the particles therein became small enough to pass through the perforations in the plates. In other words, the apparatus disclosed in the Cowles patent was a combination refiner and screen, but similar apparatus intended to perform only a screening operation is disclosed in Cram U.S. Pat. Nos. 2,489,119, 2,679,193 and 2,727,441, and in Greenwood U.S. Pat. No. 4,543,181.